Interior occupant protection has been a key focus of worldwide safety agencies and legislation for many years. Many of the structures in and around the engine compartment and front end of the vehicle are designed to maximize occupant protection in high speed impacts. While these agencies have been historically focused on occupant protection, they were also aware that there was a high fatality rate associated with or as a result of pedestrians being struck by motor vehicles. Such agencies have identified the need to reduce the number of fatalities and serious injuries to pedestrians without compromising occupant protection at a reasonable cost to the OEMs and ultimately the consumer.
New legislation—both enacted and proposed—requires that the front end of motor vehicles in some markets be less rigid, so the severity of injuries sustained by pedestrians when struck by motor vehicles is reduced. This legislation sets forth the devices, test conditions, and performance criteria for protecting both the leg and the head of the pedestrian. Areas requiring this protection include the bumper system, grille, cowl, lenses, hood, fenders, wipers, pillars and any rigid structures behind these components which may pose a significant injury risk to a pedestrian in the event of an impact.
This legislation has had an effect on the design of energy absorbing systems—requiring new concessions and compromises in styling, mass, aerodynamics, offsets, gaps, stiffness, system packaging, and other safety requirements. Conforming systems must work in concert to meet or exceed key criteria and obtain the legislated safety performance. The challenge lies in developing solutions which are highly efficient, easy to tune, low cost, light weight, and occupy the smallest footprint and volume (“real estate”).
Some pedestrian legislation requires a relatively compliant exterior surface (“skin”) that crushes in such a way that the forces experienced by the leg or the head of the pedestrian are reduced. In areas where the structure of the current components and systems cannot be easily re-engineered to meet the criteria, packaging space must be created for energy absorbing countermeasures between the impacted object and the rigid structural member which poses an injury risk to the pedestrian (see, FIG. 1). This drives the need for high efficiency energy absorbers that can absorb the desired amount of energy in the shortest distance, with the lowest mass, and lowest system cost.
One challenge with integrating a pedestrian protection energy absorber into the vehicle systems is interposing the energy absorber between layers of metal which will subsequently be required to go through a high temperature painting operation. These temperatures can be in excess of 380 F. Few plastic resins which have been traditionally formed into energy absorbing structures are capable of withstanding these temperature extremes.
Polyurethane and expanded polypropylene foam have been used in energy absorbing applications in vehicles. While these materials are easily molded to the shape and contour of the energy absorbing volume, they are relatively inefficient, heavy and costly. They also have a tendency to split or cleave when impacted against a weld flange or narrow reaction surface. In addition, their dimensional stability though the paint cycle may also be suboptimal.
Injection molded energy absorbers, commonly referred to as rib cartridges or egg crates, have also been employed as energy absorbers throughout the vehicle. While these absorbers can be molded to fit the packaging space and have high levels of efficiency, they are also heavy, difficult to tune from a performance optimization standpoint, and would need to be formed from expensive engineered resins in order to survive the paint cycle.
Some energy absorbers use high efficiency thermoformed polymeric shapes which deform and crush to absorb the impact forces. These structures may be compatible and cost effective when attached to plastic trim systems such as headliners, door panels, and bumper fascia. However, these energy absorbers are difficult to attach to the metal body and other metal components without the use of secondary attachment features. These features would be impossible to incorporate on the back side of a metal fender or a hood. While they may be able to be interposed between two layers of material, the paint process that occurs after the two layers are fused to one another reaches a temperature at which the plastic will permanently deform and lose dimensional stability unless formed out of an expensive engineered resin. When attached post-painting, these absorbers have proven to be highly efficient and cost effective, but still possess some limitations. Because plastics tend to soften at high temperatures while becoming more rigid and brittle at lower temperatures, there are some variations in performance as temperatures change under typical vehicle operating conditions. In contrast, the performance of metal is relatively insensitive to such temperature fluctuations.
A pedestrian energy absorbing bumper system described in U.S. Pat. No. 6,846,026 is composed of a low carbon steel plate formed into a U-section which absorbs energy in a repeatable manner. While this absorber is relatively efficient and has the benefit of metal construction to survive the paint process, its mass is arguably heavier than an absorber which is not constructed from a solid “plate” of material. Other sheet metal structures have also been employed as energy absorbers but traditionally have a mass and cost penalty that often outweighs their performance benefits. An energy absorber partially composed of expanded metal is disclosed in commonly owned U.S. Pat. Nos. 5,700,545 and 6,017,084.